1. Field of the Invention
The invention relates to a field strength measuring instrument comprising a loop antenna which is suitable for the simultaneous detection of E and H fields and which is divided into a first and a second half-loop by two load resistors arranged at opposite ends of the loop antenna, and comprising a circuit for determining the field strengths of the E and H fields by means of the voltages measured across the load resistors.
2. Discussion of Background
It is relatively easy to determine electro-magnetic fields at a great distance from the radiation source. In contrast, the knowledge of the near fields of the radiation source to be investigated is of much greater significance for assessing the electromagnetic compatibility (EMC) and the biological effects. This is because the probability is much higher that these fields contain inadmissible field strength peaks. However, they are also characterized by extremely complicated field structures both with standing and with propagating waves, irregular surfaces of equal phase and the like. In such fields, the Poynting vector cannot be directly derived from the magnitude of the electrical or magnetic field alone.
It is generally assumed that the susceptibility of equipment to interference from electromagnetic fields or the possibility of biological hazard is dependent on the field strength occurring. For example, the heating of lossy dielectrics (for example human tissue) and the associated risk of damage is proportional to the temporal mean value of the amount of the electric field strength squared. Similarly, the heating of partially conductive materials is proportional to the temporal mean value of the amount of the magnetic field strength squared.
Since the conventional field strength measuring instruments operate with antennas which are relatively sensitive to direction, the complicated near fields cannot be reliably measured. In addition, these measuring instruments are restricted to measuring either only the E or only the H field which, for the above reasons, is not sufficient for characterizing near fields.
A very elaborate active sensor system which, in principle, is suitable for near-field measurements, is known from the article An isolated sensor determining the Poynting vector in the near field of radiation antenna, Klaus Munter, IEEE Trans. on Instr. and Measurement, Vol. IM-32, No. 4, Dec. 1983. A measuring head comprises dipoles and magnetic loops for all three planes (x, y, z). The total of six signals (three dipoles, three magnetic loops) are amplified in parallel and converted to an intermediate frequency of 100 kHz by means of six parallel mixers. A 1-from-6 multiplexer serially switches the signals onto a similarly integrated optolink. A control device receives the optical information and forwards the signals to a vector analyzer. A computer can then be used for spatially representing E and H vector, and thus also the Poynting vector.
The system just described operates at frequencies of 500 kHz to 10 MHz and has a maximum sensitivity of 0.2 V/m and 500 mA/m. The upper limit is 50 V/m. At higher field strengths, disturbances were observed in the computer system. Since this system only processes the fields by narrow-band means, the frequency to be measured must be known. It is not possible here to measure two superimposed fields at the same time.
A wideband system for simultaneously measuring E and H field components under near-field conditions is described in the publication An electromagnetic near-field sensor for simultaneous electric and magnetic-field measurements, Motohisa Kanda, IEEE Trans. on Electromagnetic Compatibility, Vol. EMC-26, No. 3, August 1984. A loop antenna which is divided by two identical load resistors at two diametrically opposite ends is used as sensor. To determine a component, predetermined by the antenna orientation, of the E or H field, the voltages are measured across the two load resistors. To be able to measure the two voltages simultaneously without introducing any loops causing interferences with the measurements, the antenna is constructed in the manner of a thick rectangular frame, in the tubular interior of which the measuring electronics are housed.
Although the device by Kanda allows arbitrarily superimposed fields to be measured, it is restricted to measuring a predetermined directional component of the E or H field, for constructional reasons. Thus, it is not possible to measure all three components of the field vectors at the same time. In addition, it is rather large and unmanageable.